While an attempted assault in November was stopped by the government of Burundi, early in the morning on 21 December 1963, several hundred Inyenzi crossed the Burundian border and captured the Rwandan military camp in Gako, Bugesera.

The invasion and subsequent reprisals left UNAR's domestic bases of support destroyed and resulted in Rwanda becoming a de facto one-party PARMEHUTU state, while the status of the GNR was also improved.

[5] A small Hutu counter-elite began to form after World War II, consisting of persons who had been granted access to education and publications through the Catholic Church.

[10] In January 1961 thousands of Ruandan municipal officials gathered in Gitarama and, acting as a constituent assembly, voted to dissolve the monarchy and replace it with a presidential system.

[22] The party organisation was also weak, and rivalries between different factions hampered the resistance against the Rwandan government: While the UNAR exiles were led by Kigeli and reactionary monarchists, a substantial left-leaning group existed at the basis.

As more Tutsi refugees fled Rwanda, UNAR's exile ranks grew, and more systematic attempts were made to raise bands of combatants to launch raids on targeted areas.

[27][b] The Tutsi rebels reportedly also enlisted some defectors of the Armée Nationale Congolaise (National Congolese Army),[19] and were known to cooperate with Lumumbist factions in the Congo in the hopes of achieving future assistance against the Rwandan government.

[36] According to researchers Tom Cooper and Adrien Fontanellaz, the GNR proved fairly effective in repelling Inyenzi attacks in the period leading up to the Bugesera invasion.

PARMEHUTU won an overwhelming majority of the offices,[16] but the campaign revealed significant internal disagreements in the party which allowed UNAR to consolidate its domestic support.

[37] In October 1963 Kigeli gave $23,000—a portion of the money he had received from the Chinese government—to Papias Gatwa, his personal secretary, with instructions to pass it on to Rukeba, who was in the Congo at that moment.

[38] According to journalist Linda Melvern, the Inyenzi in Burundi also acquired arms with funds garnered by selling food provided by relief organisations to refugees.

"[37] Cooper and Fontanellaz argued that the decision for a large invasion, designed to win the conflict in one fell swoop, was motivated by the capture and execution of several rebel leaders in late 1963.

[17] In late November the Inyenzi in Burundi were weakened after Rukeba was arrested by local authorities, who discovered a cache of weapons in his home—purportedly stolen from Congolese rebels—and after the government seized three truckloads of arms near Bujumbura.

Upon learning of this, United Nations Commission on Human Rights (UNCHR) Representative in Bujumbura Jacques Cuenod and a group of Protestant missionaries alerted the Burundian government and frantically tried to persuade them to stop the attack.

One refugee later told UNCHR worker Francois Preziosi that Rukeba had ordered the attack after a meeting in Bujumbura during which Inyenzi leaders from other countries expressed their opposition.

Burundian Vice Prime Minister Pié Masumbuko told a Rwandan official, "Recently we have arrested people who were about to attack you and now you decide to sever economic relations with us.

[40][41] According to "reliable sources", Inyenzi leaders hoped to orchestrate simultaneous attacks on Rwanda from four different regions: Kabare, Uganda; Ngara, Tanganyika; Goma, Congo; and Ngozi and Kayanza, Burundi.

[32] Aaron Segal wrote that Rwandan leaders initially panicked when faced with invasion, fearing a multi-pronged attack supported by Burundi to restore the monarchy.

[45][46] In contrast, historian Dantès Singiza wrote that Major Camille Tulpin—a Belgian military adviser and the de facto head of the Sûreté Nationale Rwandaise—and leaders of the GNR had become informed of the Inyenzi's plans beforehand and aimed to draw them into an ambush.

[46] The Inyenzi were stopped 12 miles (19 kilometres) south of the city at Kanzenze Bridge along the Nyabarongo River by multiple units of the GNR led by Belgian officers and equipped with mortars and semi-automatic weapons.

[16] and included acts of extreme brutality; one missionary reported that a group of Hutus "hacked the breasts off a Tutsi woman, and as she lay dying forced the dismembered parts down the throats of her children, before her eyes.

In his Christmas sermon at Kabgayi Cathedral, Archbishop André Perraudin condemned the invasions and the reprisals, appealing for peace and subtly critiquing the government's actions against the political opposition, saying, "the measures of justice and legitimate defence which should be taken by those who retain power can only be approved by God if we make a generous effort of perfect fidelity to his holy laws.

"[61] Following consultations with one another, Rwanda's four Catholic bishops released a joint statement on the violence, which focused more on the "armed incursions of terrorists" who were "criminals, above all those knowing very well the incalculable evils resulting from their machinations.

"[55] In an attempt to be conciliatory towards Rwanda's Catholic leaders, Pope Paul VI sent a message to the bishops on 14 February saying that he was "profoundly saddened" by the violence and was meeting a "fervent appeal for appeasement of spirits, respect of persons, and peaceful cohabitation in fraternal charity.

"[79] In April President of the Legislative Assembly Anastase Makuza delivered a speech in Paris in an attempt to justify the massacres, saying Rwandan Hutus had acted out of a desire "not to fall victim to the fate of losers".

[67] As a result of international outcry at the massacres, the Belgian government requested the repatriation of Tulpin, Commissioner Henri Pilate, and Sub-commissioner Iréné Durieux for their reported role in the repression.

[68] Kayibanda was able to portray himself as the Father of the Nation and champion of the poor Hutu peasants, claiming to defend their hard-won gains of the revolution against the Tutsis who wanted to restore the oppressive feudalism of the old monarchy.

[76][93] Kayibanda's regime imposed ethnic quotas based on proportional representation, thus allowing Tutsis to occupy nine percent of civil service positions, but they were excluded from the political sphere.

"[69] She reasoned that it was "widely accepted" that the killings occurred due to the "extreme interpretation" by local officials of their mandate to organise self-defence groups and noted that the 6,000 Tutsis who fled to Uganda did so unhindered by the government.

[77] Historian Timothy J. Stapleton wrote, "In retrospect, the massacres of Tutsi in 1963–1964 would seem to correspond to the international legal definition of genocide; they were intentional and aimed at the extermination of at least part of a group defined along racial lines.